The replacement of incandescent bulbs by light emitting diode (LED) lighting devices reduces energy consumption due to the high efficiency of LED devices as compared to incandescent bulbs. But an incandescent bulb may be directly driven by the AC mains in contrast to conventional LED devices. It is thus conventional for an LED device to include a switching power converter such as a flyback converter to provide a regulated DC current for driving the LED. This need for a switching power converter raises costs and thus diminishes a consumer's desire to switch to LED devices. A consumer may thus continue to use incandescent bulbs, which contributes to global warming due to the increased greenhouse gas emissions from the resulting energy consumption.
To lower LED device costs, direct AC LED devices (which may also be denoted as direct AC LED bulbs) have been developed that obviate the need for a switching power converter. In a direct AC LED device, the AC mains voltage is rectified through a rectifier such as a bridge diode rectifier to produce a rectified AC input voltage. The LED in a direct AC LED device is directly driven by the rectified AC input voltage. Although no switching power converter is thus needed to convert the rectified AC input voltage into a regulated DC current/voltage, a direct AC LED device still needs a controller to control the LED power. In particular, the controller controls an LED current source in series with the LED. When the rectified AC input voltage (which may also be denoted as a post diode bridge voltage) rises above the LED threshold voltage for the LED, the controller controls the LED current by controlling the LED current source so that the LED power may be controlled accordingly.
The presence of a phase-cut dimmer switch such as a leading edge dimmer switch in household applications complicates the control of direct AC LED lighting devices. In particular, the TRIAC in a leading edge dimmer switch requires a minimum amount of holding current when conducting to prevent the TRIAC from resetting. But the post diode bridge voltage may not have exceeded the LED threshold voltage when the TRIAC begins conducting. A direct AC LED lighting device 100 that is compatible with leading edge dimming applications thus will typically include a bleeder circuit 110 as shown in FIG. 1. A dimmer switch is represented by a TRIAC that intervenes between an AC mains (AC_Input) and a diode bridge (DB). The diode bridge rectifies a phase-cut AC input from the TRIAC to produce the post diode bridge voltage carried on a power rail 105. Bleeder circuit 110 couples to power rail 105 to conduct a holding current into ground. A controller (I_CNTRL) controls a current through an LED string by controlling an LED current source 115.
Bleeder circuit 110 includes a controller (I_Bleeder) that controls the bleed current. In particular, the controller switches a bleeder circuit current source that conducts a bleed current from power rail 105 to ground while the TRIAC is conducting and the LED is not conducting. In a leading edge dimmer switch, the phase angle over which it does not conduct begins at the front edge (zero crossing) of each half cycle for the AC input voltage. Each half cycle of the post diode bridge voltage will have a phase cut portion in which the leading edge dimmer switch does not conduct depending upon the dimmer switch setting by the user. Outside of the phase cut portion in each half cycle, the post diode bridge voltage will behave normally so that it follows a sinusoidal profile from each zero crossing of the post diode bridge voltage. In particular, the post diode bridge voltage from a leading edge dimmer switch will have a rising edge following the phase cut portion. From this rising edge, the post diode bridge voltage will continue sinusoidally increasing until it reaches the LED threshold voltage. At that point, the LED will conduct the minimum holding current to prevent the TRIAC from resetting. The bleeder controller thus may control the bleeder current source to conduct only while the TRIAC conducts and the LED is not conducting.
The resulting bleeder current waveform as a function of the post diode bridge voltage waveform is shown in FIG. 2. In each half cycle of the post diode bridge voltage, the LED of FIG. 1 will not conduct until the post diode bridge voltage exceeds the LED threshold voltage. Each half cycle starts from a zero crossing point such as time t0 for an initial half cycle. The TRIAC of FIG. 1 does not conduct until its phase cut portion has expired such as at time t1. From time t0 to t1, the post diode bridge voltage is thus discharged. At time t1, the rising edge of the post diode bridge voltage is passed due to the TRIAC beginning to conduct. The bleeder current must thus flow from time t1 until a time t2 when the LED threshold voltage is satisfied.
Accordingly, there is a need in the art for direct AC LED devices with reduced jitter bleeder current losses in the presence of a phase-cut dimmer switch.